A method of forming a Cu—In—Ga—Se compound film by laminating a Cu—Ga film and an In film, which are formed by sputtering, and heat-treating the obtained laminated film in a gas atmosphere containing Se has conventionally been used to form a light-absorbing layer of a CIGS type thin film solar cell (for example, Patent Document 1). However, in the film formation method, the respective film formation chambers of a Cu—Ga binary alloy and In, and sputtering target materials are necessary, and a heat-treating furnace is further necessary for conducting heat treatment in a Se atmosphere. Thus, production costs were high.
In view of the above, development of a printing method using a Cu—In—Ga—Se compound powder, a deposition method using a Cu—In—Ga—Se compound or a sputtering method using a Cu—In—Ga—Se compound sputtering target material is being performed to efficiently form a Cu—In—Ga—Se compound film. However, when a Cu—In—Ga—Se compound is tried to be prepared by the conventional powder sintering method (for example, a method of preparing the respective powders of Cu, In, Ga and Se, and then sintering) or a melt casting method, there was a danger that In rapidly reacts with Se simultaneously with melting of In, and explosion occurs. To avoid the danger, for example, Patent Document 2 discloses a method for producing a Cu—In—Ga—Se quaternary compound molten metal by introducing Cu in Se to prepare a Cu—Se binary alloy molten metal, introducing In in the Cu—Se binary alloy molten metal to prepare a Cu—Se—In ternary compound molten metal, and introducing Ga in the Cu—Se—In ternary compound molten metal obtained. However, even in this method, there is a possibility of explosion such that if Se remains as a single phase in the Cu—Se binary alloy molten metal, Se rapidly reacts with In, and there is still room for improvement in safety and stability. Furthermore, for example, Patent Document 3 discloses a method for preparing a hot-pressed body by preparing a Cu—Se binary alloy powder, a Cu—In binary alloy powder, Cu—Ga binary alloy powder and a Cu—In—Ga ternary alloy powder, mixing those powders, and hot-pressing the resulting mixture. This method is a method of simultaneously conducing making in the form of a compound and sintering by hot-pressing a mixed powder. Therefore, considering that a gas is generated in a chamber when making in the form of a compound, a temperature of a hot press cannot be increased so much (for example, about 140° C.). As a result, there is high possibility that relative density of a hot-pressed body obtained cannot be increased and mechanical strength is insufficient, or cracks occur during hot-pressing or during processing after hot-pressing.